configure.ac: version bump to 2.3.1

Signed-off-by: Tadeusz Struk <tadeusz.struk@intel.com>
diff --git a/configure.ac b/configure.ac
index cccb62a..f2661b6 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4,7 +4,7 @@
 # All rights reserved.
 
 AC_INIT([tpm2-tss],
-        [2.3.1-rc0],
+        [2.3.1],
         [https://github.com/tpm2-software/tpm2-tss/issues],
         [],
         [https://github.com/tpm2-software/tpm2-tss])